Thats because the base reward is fixed at 2 milli Dollar (0.002$). If the Bitcoin price in Dollar raises, the base reward in Satoshi drops. Also depending how much you wager, deposit, etc the base reward is further reduced. Currently you should get maximal 5 Sat/Roll as base prize. If you dont wager, buy lottery tickets, have a low balance, you get less then 5 Sat/Roll.